1. Field of the Invention
This invention relates generally to integrated circuit packages, and more particularly to a method for making multi-layer ceramic packages by attaching contact pins to a ceramic substrate after the application of an intermediate metal layer and an outer metal layer to the multi-layer ceramic package.
2. Related Art
In the electronics industry, integrated circuits are commonly encased in packages made of plastic or ceramic. A ceramic integrated circuit package comprises a ceramic substrate including contact pins to connect an integrated circuit enclosed within the package to external circuitry. The package is hermetically sealed (for example, with a ceramic lid).
Fabricating a ceramic integrated circuit package requires a number of steps in order to properly adhere conductive metal patterns and contacts to the ceramic package. Such metal patterns and contacts are necessary in order to provide electrical paths between external circuitry and an integrated circuit package affixed within the ceramic package.
Typically, a metal such as tungsten is used to form metallic wire bonding areas, die attach area, and external bonding pads on the ceramic. Tungsten is known to directly adhere to ceramic materials. However, other materials known in the art can be used in place of tungsten.
In order to provide corrosion resistance, it is highly desirable to coat or plate tungsten wire bonding areas, die attach area, and external bonding pads with an inert metal, such as gold. However, gold does not readily adhere directly to tungsten. Therefore, it is common to use an intermediate metallic layer, such as nickel, to plate tungsten bonding areas and then to overplate the intermediate layer with gold.
The current method of making multi-layer ceramic packages, as illustrated in FIG. 1, consists of five basic steps:
step 1: firing a ceramic substrate 10 with metallized (e.g., tungsten) sealing pad area 11, wire bonding areas 12, die attach area 13, and external bonding pads 14 (FIG. 1A); PA1 step 2: plating the sealing pad area 11, wire bonding areas 12, die attach area 13, and external bonding pads 14 with a first intermediate metal layer A (e.g., nickel) (FIG. 1B); PA1 step 3: brazing contact pins 15 to the plated external bonding pads 14 on the ceramic substrate 10 (FIG. 1C); PA1 step 4: plating the metallic portions of the ceramic substrate 10, including the contact pins 15, with a second intermediate metal layer B (e.g., nickel) to promote adhesion of a subsequent layer of gold (FIG. 1D); PA1 step 5: plating the metallic portions of the ceramic substrate 10, including the contact pins 15, with an outer metal layer C (e.g., gold) (FIG. 1E).
The resulting package has an intermediate metal layer B and an outer metal layer C on the contact pins 15, which may not be necessary for subsequent solder dipping operations, In such a case, this extra plating step costs a considerable amount due to the material and handling costs. The cost is especially significant since the number of contact pins that a multi-layer ceramic package contains has increased in the past few years from 18-24 pins to over 100 pins.
Therefore, it is desirable to provide a method for making multi-layer ceramic packages that reduces the amount of metal plating on contact pins. The present invention accomplishes this goal.